Silver halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver.
In color photographic elements a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye forming compound called a coupler. The silver and unreacted silver halide are then removed from the photographic element, leaving a dye image.
In either case, formation of the image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and coupler. Thus, gelatin, and similar natural or synthetic hydrophilic polymers, have proven to be the binders of choice for silver halide photographic elements. Unfortunately, when gelatin, and similar polymers, are formulated so as to facilitate contact between the silver halide crystal and aqueous processing solutions, they are not as tough and mar-resistant as would be desired for something that is handled in the way that an imaged photographic element may be handled. Thus, fingerprints can easily mark the imaged element, it can be scratched or torn and it can swell or otherwise deform when it is contacted with liquids.
There have been attempts over the years to provide protective layers for gelatin based photographic systems that will protect the images from damages by water or aqueous solutions. U.S. Pat. No. 2,173,480 describes a method of applying a colloidal suspension to moist film as the last step of photographic processing before drying. A series of patents describes methods of solvent coating a protective layer on the image after photographic processing is completed and are described in U.S. Pat. Nos. 2,259,009, 2,331,746, 2,798,004, 3,113,867, 3,190,197, 3,415,670 and 3,733,293. U.S. Pat. No. 5,376,434 describes a protective layer formed on a photographic print by coating and drying a latex on a gelatin-containing layer bearing an image. The latex is a resin having a glass transition temperature of from 30.degree. C. to 70.degree. C. The application of UV-polymerizable monomers and oligomers on processed image followed by radiation exposure to form crosslinked protective layer is described in U.S. Pat. Nos. 4,092,173, 4,171,979, 4,333,998 and 4,426,431. One drawback for the solvent coating method and the radiation cure method is the health and environmental concern of those chemicals to the coating operator. The other drawback is that these materials need to be coated after the processing step. Thus, the processing equipment needs to be modified as well as the personnel running the processing operation need to be trained. In addition, several lamination techniques are known and practiced in the trade. U.S. Pat. Nos. 3,397,980, 3,697,277 and 4,999,266 describe methods of laminating polymeric sheet film on the processed image as the protective layer. U.S. Pat. No. 5,447,832 describes the use of a protective layer containing a mixture of high and low Tg latices as the water-resistant layer to preserve the antistat property of the V.sub.2 O.sub.5 layer through photographic processing. This protective layer is not applicable to the image formation layers since it will detrimentally inhibit the photographic processing. U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the emulsion, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not desirable for large-scale application. U.S. Pat. No. 3,443,946 provides a roughened (matte) scratch-protective layer, but not a water-impermeable one. U.S. Pat. No. 3,502,501 provides protection against mechanical damage only; the layer in question contains a majority of hydrophilic polymeric materials, and must be permeable to water in order to maintain processability. U.S. Pat. No. 5,179,147 likewise provides a layer that is not water-protective. However, all these techniques need to be carried out after the image has been formed, which adds a large cost to the final imaged product.
Thus, the ability to provide the desired property of post-process water/stain resistance of the imaged photographic element, at the point of manufacture of the photographic element, is a highly desired feature. However, in order to accomplish this feature, the desired photographic element should be permeable to aqueous solutions during the processing step, but achieve water impermeability after processing, without having to apply additional chemicals or to substantially changed the chemicals used in the processing operation. U.S. Ser. No. 09/235,436 discloses the use of a processing solution permeable overcoat that is composed of a urethane-vinyl copolymer having acid functionalities. However, the limitation of coating such a polymer is that, at coverages desired for durability, the overcoat tends to exhibit defects such as cracks which are formed during the coating process. In addition, the presence of the overcoat causes a slight decrease in the permeation and reaction rates of the developer with the light sensitive emulsions in the underlying layers, resulting in a greater possibility of variability in image-quality.
Therefore there remains a need for a protective overcoat for an imaging element that can be coated free of defects such as cracks and, which at the same time will not significantly reduce the rate of reaction of the developer with the underlying emulsions and will also provide a water impermeable and durable overcoat after the processing step.